The need for nanodevices in electronic components is increasing logarithmically. One particular need is for small, reliable energy sources to drive these components. One such device is a photoelectrochemical cell.
A photoelectrochemical cell is one where an electrical potential is generated between two electrodes by the action of light on an electrolyte. The efficiency of such a device can be high compared to conventional silicon photocells using semiconductors. Most photoelectrochemical cells use a transition metal complex, often called “dyes”, along with a metal oxide particle, such as those described in U.S. Pat. Nos. 5,350,644, 5,441,827 and 5,728,487. The efficiency can be even further increased by using the unique electronic properties of CNTs in place of the metal oxide particle.
Carbon nanotubes (CNT) have been the subject of intense research since their discovery in 1991. CNTs possess unique properties such as small size, considerable stiffness, and electrical conductivity, which makes them suitable in a wide range of applications, including use as structural materials and in molecular electronics, nanoelectronic components, and field emission displays. Carbon nanotubes (CNTs) are rolled up graphene sheets having a diameter on the nanometer scale and typical lengths of up to several micrometers. CNTs can behave as semiconductors or metals depending on their chirality. Additionally, dissimilar carbon nanotubes may contact each other allowing the formation of a conductive path with interesting electrical, magnetic, nonlinear optical, thermal and mechanical properties. Carbon nanotubes may be either multi-walled (MWNTS) or single-walled (SWNTs), and have diameters in the nanometer range.
It is known that single walled carbon nanotubes are sensitive to their chemical environment, specifically that exposure to air or oxygen alters their electrical properties (Collins et al. (2000) Science 287:1801). Additionally, exposure of CNTs to gas molecules such as NO2 or NH3 alters their electrical properties (Kong et al. (2000) Science 287:622). Thus chemical gas sensors can be designed, based on how they influence the electrical properties of carbon nanotubes such as described in DE10118200.
Carbon nanotubes have been used in electrocatalysis. Microelectrodes, constructed of multiwalled carbon nanotubes, were shown to provide a catalytic surface for electrochemical reduction of dissolved oxygen, potentially useful in fuel cell applications (Britto et al. (1999) Advanced Materials 11:154). A film of single walled carbon nanotubes functionalized with carboxylic acid groups on a glassy carbon electrode showed electrocatalytic behavior with several redox active biomolecules, involving reduction of the carboxylic acid groups (Luo et al. (2001) Anal. Chem. 73:915). Toluene-filled multiwalled carbon nanotubes as a film on a gold electrode surface were shown to respond better to electroactive biomolecules than empty carbon nanotubes (Zhang et al. (2003) Electrochimica Acta 49:715).
The ability to alter the electrochemical characteristics of carbon nanotubes suggests applications involving redox reactions. For example, Larrimore et al (Nano Letters (2006), 6(7), 1329-1333) teach the use of single-walled carbon nanotube transistors to measure changes in the chemical potential of a solution due to redox-active transition-metal complexes. Similarly WO 2004112163 (Sainte et al.,) teach photovoltaic cells comprised of carbon nanotubes (single-walled or multiple-walled), conjugated polymers, and at least one organic pigment or dye. Specifically, the photovoltaic cell consists of one or more conjugated polymers, which function as an electron donor, and the carbon nanotubes containing the pigment or dye, which functions as the electron acceptor. Commonly owned and copending U.S. Ser. No. 11/241,515 teaches that changes in the redox potential of a solution will produce alterations in the electrochemical properties of a carbon nanotube that may be quantified for analyte detection.
The ability to use carbon nanotubes as elements in redox reactions makes them ideal for photocell applications. A need exists therefore for small photocells that operate on the basis of redox reactions. Applicants have solved the stated problem via the development of a photochemical cell that comprises a photochemical system comprising a transition metal complex in association with a population of carbon nanotubes and an electron acceptor. In the presence of light the system functions to generate an electron flow to the electron acceptor for energy production.